CD274 and plasma cell myeloma: PD-L1 expression on MM cells is upregulated by the IL-6 signal cascade through STAT3, MEK1/2, and JAK2, and by IFN-γ produced by CTLs and NK cells through the MEK/ERK pathway [85,86], suggesting that PD-L1 expression can be induced in the myeloma microenvironment.